An organic EL device is a surface emitting device having an organic thin film sandwiched by a pair of electrodes, and it has characteristics such as being thin and light in weight, having a wide viewing angle, having high speed responsiveness, etc. and thus is expected to be applied to various display devices. Further, recently, its practical application to e.g. the display of a mobile phone has partly started. Such an organic EL device is one which utilizes light emitted at the time when holes injected from an anode and electrons injected from a cathode are recombined in a luminescent layer, and its prevailing structure is a multilayer laminate type having a hole transport layer, a luminescent layer, an electron transport layer, etc. laminated. Here, a charge transport layer such as a hole transport layer or an electron transport layer, does not emit light by itself, but plays roles to facilitate charge injection to the luminescent layer and to confine the charge injected to the luminescent layer and the energy of excitons formed in the luminescent layer. Thus, the charge transport layer bears very important roles to reduce the driving voltage and to improve the luminous efficiency of an organic EL device.
As the material for a hole transport layer, an amine having a proper ionization potential and hole transporting ability is employed, and for example, 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (hereinafter abbreviated as NPD) is well known. However, the driving voltage, luminous efficiency and durability of the device employing NPD as the material for a hole transport layer, are not sufficiently good, and it is desired to develop a new material. Further, in recent years, development of an organic EL device employing a phosphorescent emitting material for a luminescent layer has been in progress, and in the device employing phosphorescent, a material for a hole transport layer having a high triplet level is required. Also from the viewpoint of the triplet level, NPD is not adequate, and for example, it has been reported that the luminous efficiency decreases in an organic EL device wherein a green-emitting phosphorescent emitting material and NPD are combined (see e.g. Non-patent Document 1).
Under such circumstances, recently, there have been reports on an amine compound having a carbazole ring introduced to its molecule, as a material for a hole transport layer. Specifically, reports on a 2-aminocarbazole compound may be mentioned (see e.g. Patent Documents 1 and 2). The 2-aminocarbazole compound has an ionization potential equal to or higher than NPD and a triplet level higher than NPD. Therefore, it is known that the 2-aminocarbazole compound usually tends to exhibit higher luminous efficiency than NPD in a device employing a green phosphorescent material.
Further, a 3-aminocabazole compound having carbazole combined with dibenzothiophene or dibenzofuran, has also been disclosed (see e.g. Patent Document 3).
However, with respect to an organic EL device, further reduction of the driving voltage, higher luminous efficiency and longer lifetime are desired, and it is desired to develop a material for a hole transport layer for such purposes.